Oxidative Phosphorylation Flashcards
Revision
What happens to the electrons after glycolysis and the TCA cycle?
From one glucose molecule, the reactions of glycolysis, pyruvate dehydrogenase complex, and the TCA cycle produce
- 10 NADH + H+
- 2FADH2
Each one of them carries two high-energy electrons.
We still don’t know how these molecules are re-oxidised so that they can be reused.
What is the role of electrons in oxidative phosphorylation?
Electrons from NADH and FADH2 are used to reduce O2 and H2O.
Their energy is used to pump protos (H+) from the mitochondrial matrix to the intermembrane space
- pH decreases in intermembrane space, increases in the matrix
Protons flow back across the membrane, following their concentration gradient.
Energy of proton flow is used to phosphorylate ADP to ATP.
(This movement of protons is not energetically favourable, so something needs to provide the energy.
There is enough in each of these electron to pass down the chain and transfer enough energy to pump the protons across the membrane.
This back pressure is released by the protons flowing through molecules of ATPsynthase).
How do the NADH from the cytoplasm cross the cell membrane.
During glycolysis, 2NADH are formed in the cytoplasm.
NADH cannot cross the inner mitochondrial membrane.
It cannot be re-oxidised to NAD+ directly using the electron transport chain.
The glycerol-3-phosphate and malate-aspartate shuttles overcome this problem.
1. NADH from glycolysis I used to generate malate from oxaloacetate in cytosol.
2. Malate transporters transfer malate to mitochondrial matrix.
3. Malate conversion to oxaloacetate in TCA cycle generates NADH in addition to the malate that arises from fumarate.
(NAD is turned into NADH in the cytoplasm.
The NAD is recycled in the cytoplasm.
Malate is reduced to form oxaloacetate.
This is then turned into aspartate).
What are the redox potentials of high-energy electrons?
In oxidative phosphorylation, the electron transfer potential of NADH+ and FADH2 is converted into the phosphoryl transfer potential of ATP.
Phosphoryl transfer potential can be measured by the free energy change, DeltaGo’, for the hydrolysis of ATP.
Electron transfer potential is measured by the redox potential (or reduction potential), E’o, of a compound.
What is the standard redox potential, what does a negative standard redox potential mean and what is the standard free energy change proportional to?
The standard redox potential E’o of a (reduced) substance X is a measure for how readily X donates an electron (in comparison with H2).
X- -> X +e-
A negative E’o means that the reduced form of X has a lower affinity for electrons than H2, a positive E’o means the opposite.
The standard free energy change is proportional to the change in standard redox potential and the number of electrons transferred.
(If you readily give away your electrons and you have a lot of electrons to give, you have a large change in free energy).
How much energy can be released by the reduction of O2 by NADH?
The reduction of O2 by NADH is the driving force of oxidative phosphorylation.
Redox potentials:
1/2O2/H2O +0.82V
NAD+/NADH -0.32V
The standard free energy change for the reaction
1/2O2 + NADH + H+ -> H2O +NAD+
is DeltaGo’ = -220.1kJ/mol
The standard free energy change for the hydrolysis of ATP is DeltaGo’ = -31.4kJ/mol
What is the process of oxidative phosphorylation?
Coupling of respiration to ATP synthesis.
Chemiosmotic hypothesis (Peter Mitchell, 1961).
Consists of two stages:
1. electron transport
- electrons flow from NADH and FADH2 to O2
- Respiratory chain
- Energy is used to pump H+ out of the mitochondrial matrix.
2.ATP synthesis
- Electrochemical gradient of H+ across mitochondrial inner membrane.
- Energy stored in this gradient can be used to synthesise ATP
Electron transport and ATP synthesis are catalysed by separate proton pumps.
What is the process of the electron transport chain?
4 multisubunit complexes
- in inner mitochondrial membrane
Electrons from NADH enter at complex I.
Electrons from FADH2 enter at complex II
- Complex II is part of the TCA cycle!
Electrons re handed to carriers with increasingly positive (oxidising) redox potentials.
Electrons are ultimately transferred onto O2 to form H2O
- that’s why we need to breathe oxygen
(You want to avoid large free energies because they are difficult to harness and create a lot of heat that would cook the cell).
What are cytochromes?
Cytochromes are proteins which contain a haem group as a functional co-factor.
Haem contains an Fe(II) ion which can take up and release electrons.
What is the role of the H+ pump?
Transfer of electrons through the respiratory chain is coupled to transport of H+ from the mitochondrial matrix to the intermembrane space.
Three of the four respiratory complexes pump H+.
What is the role of the electrochemical gradient?
More protons in the intermembrane space than in the matrix.
Forms an electrical field with the matrix side more negative.
Protons “want” to flow back into the matrix.
Flow back into the matrix is coupled to ATP synthesis.
Protons are pumped across this membranes as electrons flow through the respiratory chain.
(High concentration - low pH.
Low concentration - high pH.
They want to flow because of their charge and they are attracted to the negative charge on the other side of the membrane).
Why is there a flow of protons across the membrane in the electron transport chain?
Respiratory chain pumps protons into intermembrane space.
Protons flow back through ATP synthase.
Two separate proton pump systems.
(If the protons cannot cross the membrane, you die).
How is the role of ATP synthase carried out?
ATP synthase is also called
-Mitochondrial ATPase
- F1F0ATPase (1 and 0 are subscript).
F1 subunit protrudes into mitochondrial matrix.
F0 subunit is a hydrophobic complex in the inner membrane
- contains the proton channel
a, b, alpha, beta and gamma subunits form stator.
c, gamma and E subunits form rotor.
Flow of protons turns the rotor.
Conformational changes lead to ATP synthesis.
(As the protons flow, they cause a rotation of some of the subunits in comparison to other subunits).
How can oxidative phosphorylation be inhibited?
Electron transport chain can be inhibited at many stages
- cyanide, azide and CO inhibit transfer of electrons to O2
- no proton gradient can be formed
- no ATP can be synthesised
What are Uncoupling Proteins (UCPs): Physiological Regulators of Proton Leak
UCP-1: First UCP isoform to be discovered. Found in brown fat tissue - e.g. in newborns, hibernating mammals.
Needs free fatty acids for activation.
Primary role in adaptive non-shivering thermogenesis.
(Cold regulates non-shivering thermogenesis)
